Demon

In the universe of Devil May Cry, the two dominant species are humans and demons. Demons are the primary lifeforms of the Demon World, although their exact nature is unclear. Demons have a range of shapes and sizes, and usually follow a power-based hierarchy.

Powers
Demons are naturally stronger than normal humans, and are able to perform superhuman and supernatural feats. The abilities of demons are greatly varied. Demons usually seems to possess powers that are based on a certain element, such as fire or lightning. After a demon is killed, it is sometimes possible to absorb that demon and manifest their power or transform it into a weapon known as a "Devil Arm". Though demons naturally are more powerful then humans, and possess abilities beyond that of any human, that does not prevent humans from being able to fight and even kill a demon (though humans with this ability seem to be very rare and have spent much of their life training to do so). In Devil May Cry 4, Dante suggests that when a demon becomes selfless or loves, they can achieve greater power.

Nature
The nature of demons is unkown. Though they are generally very selfish and cruel, and often seek to increase their own power or influence however evil is not a universal trait for them nor is it part of their nature. Very rarely do demons ever show remorse or pity, particularly when regarding humans. However, there are some cases where they will show regret or remorse, especially if a close friend, relative or loved one of theirs dies.

Demons and Humans
Demons view themselves as superior to humans, and as such wish to rule the human world as well as their own. It is implied that before Sparda's uprising, the demons held dominion over the human world and there were many gates and portals between the two realms. Ever since Sparda sealed off the demon world from the human world, demons have been attempting to regain dominion over the humans. Although demons are considered "evil", Dante has also described humans as sharing this quality, and that in many cases humans are even worse, a trait that was backed up by the actions of corrupt humans such as Arius.

In the anime series, novels, and in DmC, it is hinted that demons have disguised themselves as humans and have gained an incredible amount of influence in the criminal underworld while some reach the point to where they gain influence over humans behind their false image. Most of these demons who have gotten in have taken on the jobs of mafia members, crime bosses, corrupt authority figures (police officers and religious leaders), and even the wealthy upperclass citizens.

However, there are some demons who enter the human world to live with mortals for reasons such as starting a new life free from their pasts or to live with humans in peace while keeping their own true natures hidden.

Angels
The powerful demons were once angels in service to divine forces, therfore, angels are the same thing as demons. The Fallen is a demon reminiscent of an angel who has fallen from grace for deceiving her victims. The Order of the Sword transforms its members into angels with the ascension ceremony, which are actually humans with demon abilities. Its possible that angels are in fact demons...such as Sparda...who rebelled against Mundus and joined the humans, many of whom were worshiped as gods.